Some people say that two is always better than one, but in some cases one subwoofer can be better than two. When referring to women, they might be right, however this isn't always true when it comes to car audio. Most people will try to cram two or even three subwoofers into the trunk of their car because they think it will be louder, hit harder, or maybe because they think it'll impress their buddies. But, here's the reality:
If you get one high quality subwoofer, an enclosure that matches the subwoofer's specs, and a quality amplifier that will supply it with ample power to go with it, you can make one sub hit just as hard as two, if not harder. The fact is that most car trunks (not all) do not have adequate space for two subwoofers nor the correct amount of square footage needed for it/them to operate at maximum efficiency and power. There is also a possibility for multiple subwoofers to fight against each other depending on the subwoofers' enclosure and the way in which they are facing. They can even cancel each other's sound waves out in some cases.
The bottom line is: if you are looking for lots of bass and you want to impress your friends, you can do it with one subwoofer and the correct set up.